The invention is related to a rolling element bearing comprising an inner ring and an outer ring each provided with a raceway, at least one of said rings having land regions on opposite sides of the respective raceway, and a series of rolling elements which are in rolling contact with the raceways and which are mutually spaced by a cage, said cage engaging the land regions of said ring, said land regions being coated.
Such rolling element bearing is known from EP-B-531082. The rolling elements are spaced by a cage, which rides on the lands provided on one of the rings next to the raceway thereof. Such bearing has the advantage that the cage is guided by the ring in question, which improves the dynamic behaviour of the ring and reduces whirl instability.
According to EP-B-531082, a hard coating is applied to each land of the ring, but not to the raceway thereof. Thereby, flaking off under the influence of the locally very high compressive forces which occur in the rolling contacts between the rolling elements and the raceways, is to be avoided.
The object of the invention is to provide a rolling element bearing in which the problem of the coating flaking off will not occur, which enables a relatively cheap manufacturing method, and whereby a proper guidance of the cage is maintained. This object is achieved in that said least one ring is coated with a wear and friction reducing, elastic coating over its full surface.
The coating applied both on the lands as well as on the raceway and the other surfaces of the bearing ring elastic, several advantages are obtained. First of all, it provides a desired guidance of the cage, such that swirl instability is avoided.
Secondly, the coating will not flake off from the raceway, in particular in case the coating has a thickness which is less than the depth beneath the raceway at which the shear stresses resulting from the rolling motion of the rolling elements, are maximal. A relatively thin layer can deform easily and follow the elastic deformation of the steel base material, when the ball rolls over the raceway. In contrast, in the case of a relatively thick coating, the stresses in the coating layer will be high and spalling or flaking off is more likely. Also a thick elastic coating is more difficult to adhere to the steel than a thin coating due to internal stresses in the coating layer.
As a consequence, according to a third advantage the bearing can be manufactured in an economic way. The layer thickness can be very small. Moreover the full surface of the bearing rings may be coated which is easier than coating only the lands thereof.
An advantage of a very small coating thickness is that the topography of the steel raceway is maintained, as a result of which the dynamic behaviour of the bearing is not impaired.
According to a preferred embodiment, the coating comprises a diamond-like carbon coating. In particular, the coating may comprise a metal containing diamond-like carbon, for instance alternating layers of metal such as W or WC and a hard morphous coating, such as diamond-like carbon.
According to a further embodiment, the coating may comprise of boron-nitride (BN), chromium nitride (CrN), hafnium nitride (HfN) niobiumnitride, carbonnitride or any other nitride, oxide such as boron oxide, or carbide or sulphide coating.
The maximum coating thickness may be about 2 xcexcm; preferably, the maximum coating thickness is 1 xcexcm.
Reference is made to U.S. Pat. No. 5,112,146, related to a rolling element bearing, the rings of which have a very hard, low fiction coating. Said coating however does not act as a support for a cage, nor is the entire surface of the bearings rings fully coated.
The coating can be deposited by means of physical vapour deposition (PVD), chemical vapour deposition (CVD) or pulsed laser deposition (PLD) techniques or through surface treatment like Ion Implantation or laser cladding or glazing.
In addition the hard coating can be further enhanced by another top coating that creates solid lubrication, thereby creating a very smooth interface between cage and ring through transfer of a solid lubricant layer to the counterface.
Such coating can be MoS2 or WS2 e.g. for applications where dry running in the rolling contact is possible.